If the Oz spec rear ends are the same as the US ones, the 6x4 usually has a pair of 20,000lb Meritor axles, recommended practice for singling them out is to start with a pair of 23,000lb axles, lose the front one and move the rear one forward to the point that you can use a factory 4x2 prop shaft.
I wouldn’t worry too much about using a 20,000lb axle, if you’re downsizing from 6x4 to 4x2 then its usually because the loads are light and there’s a decent amount of over engineering in an axle to take driver abuse or accidental shock into account and you already own the axle, so why not use it until you need to replace it, if the worst case happens.
I would make sure it had a diff lock though and I would probably have the ECU set up for progressive shifting, to make things a little more gentle on the oily bits.
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